Fig. 4From: Relationship between wave-like auroral arcs and Pi2 disturbances in plasma sheet prior to substorm onsetThe temporal evolution of the azimuthal mode number spectrum of the phase space density (PSD) of the wave-like auroral arc intensity structure. The PSD is defined to be the square of the arc intensity averaged over the arc length. Note that the intensity color scale is different in the two panelsBack to article page